One purpose of the transaction log file (*.log) is to safeguard the database from corruption. All changes to the database are first stored in the log file. At periodic intervals the changes transfer to the database file. This process allows the engine to keep the database closed, preventing corruption to the database file.
When the database engine shuts down unexpectedly (such as during a power outage), corrupted information may be stored in the log file. If this occurs, the database engine does not start when power is restored to the server. Error messages such as "Unable to start database engine" and "Assertion Failed" can indicate corrupted log files.